1. Field of the Invention
The present invention relates to a scheduling device and cell communication device. In ATM communication, for example, the present invention can be applied to a scheduling device in which cell output control is performed for each individual service class or connection and to an ATM node device provided with such scheduling device.
2. Description of the Related Art
An ATM communication system is a system in which processing of voice, images, data, or various other types of service information is performed in integrated fashion. Performing such multiple tasks creates a problem of interference of bands of use between different services.
In the ITU-T Recommendations made by the ITU-TS (International Telecommunication Union Telecommunication Standardization Sector), ATCs (ATM Transfer Capabilities) known as DBR (deterministic bit rate), SBR (statistical bit rate), ABR (available bit rate), UBR (unspecified bit rate) and GFR (guaranteed frame rate) are standardized (defined) in accordance with the traffic characteristics of the service. Also, the service classes (QoS) known as class 1, class 2, class 3 and U class are standardized in accordance with service quality such as the cell loss rate and cell delay variation. Consequently, in an ATM communication network, interference of bands of use occurs between respective ATC/QoS.
In order to avoid such band interference, ATM node devices are provided with a scheduling device located between ATC/QoS or between connections and cell reading is performed in accordance with the schedule determined by the scheduling device.
Conventional scheduling devices usually avoid the band interference by providing a cell buffer for each ATC/QoS or for each connection and performing the cell reading at specified intervals. Various designs and schemes have been developed for such cell buffer and scheduling. For example, Japanese Patent Kokai No. 11-355304 discloses a scheduling device that has a timing table for each QoS such that reading of cells from a cell buffer is performed in accordance with this timing table, and the timing table is updated when the cell reading takes place.
FIG. 2 of the accompanying drawings shows a timing table used by a conventional scheduling device. As shown in this drawing, the timing table has binary information storage regions from T1 to Tx for each QoS and 1 is set in each QoS at the time when the cell is to be read.
For example, when the time T2 is reached a cell is read from the cell buffer QoSN where 1 is set in the timing table. Immediately, the binary information at the time T2 of QoSN is cleared (set to 0), and 1 is set at the time of T2 plus the reading interval. For example, if the reading interval of QoSN is 5, 1 is set at T7.
Likewise at time T3 cell reading is performed from the cell buffer of QoS2, the binary information of time T3 of this QoS2 is cleared (set to 0), and 1 is set at the time of T3 plus the reading interval of QoS2.
This action is repeated at T4, T5, T6 . . . . A description of competition control when 1 is set in a plurality of QoS at the same time will be omitted.
In this way, the conventional scheduling system reads a cell (or cells) from the cell buffer of each ATC/QoS under the control of a timing table prepared on the basis of the desired (designed) time sequence.
However, with the scheduling system shown in Japanese Patent Kokai No. 11-355304, storage regions of (QoS number N)×(time Tx) must be provided in the timing table to follow the desired time sequence. Also, this Tx depends on the cell-reading intervals of the accommodated services, and therefore needs to be the maximum reading interval (lowest rate) among the various cell-reading intervals (or the accommodated services).
As a result, there is a problem that the storage medium needed for the scheduling function must have a large capacity. Consequently, there is a problem that the entire ATM node apparatus which includes the scheduling device is also increased in size.
For example, in the case of ITU-T I. 371, the cell rate (A: units [cells/second]) is given by a floating point representation; accordingly if a minimum settable rate, i.e., 1 [cell/second], should be provided and the physical rate of the communication path is assumed to be 622.08 [Mbit/s], the reading interval becomes about 1.5×106 [cells]. Therefore, even if N is 1, a storage region capacity of 1.5 M bits is necessary.
Consequently, there is a demand for a scheduling device that is capable of greatly reducing the storage region capacity required for the scheduling function.